1. Field of the Invention
The present invention relates to a frame guard system for preventing a failure of framing or an out-of-frame state occurring after the framing takes place.
2. Description of the Related Art
Conventionally, in such a frame guard system in a receiver of a mobile telephone, the number of frame guard stages has been fixed. The frame guard is divided into a forward guard and a backward guard. When the guard stage number in the backward guard is small, the framing is quickly done but easy to be in the out-of-frame state after the framing takes place. When the guard stage number in the backward guard is large, the framing by a pseudo frame synchronization signal does not occur but the framing tends to be delayed.
When the forward guard stage number is small, the presence of a small number of the out-of-frame bits causes the signal to tend to be out-of-frame, with a result that the communication becomes unstable. When the forward guard stage number is large, the frame is prevented from being out-of-frame for even a small number of the out-of-frame bits, but a frame alignment signal tends not to be stopped even in the out-of-frame state. In this case, the frame alignment signal refers to a signal that a frame guard system is to externally output as its final guard result.
In particular, when such a moving object as a mobile telephone moves into a tunnel or the shade of a mountain where the moving object can weakly receive an electromagnetic wave signal, a level in its received signal is abruptly decreased, which leads to the fact that the out-of-frame state is unfavorably caused immediately after the framing takes place.
In other words, the prior art frame guard system has had such a disadvantage that, since the respective guard stage numbers of the forward guard and the backward guard are fixed, the framing is deteriorated or the out-of-frame is unfavorably caused immediately after the framing is carried out, depending on a situation.
As explained above, the prior art frame guard system has such a problem that, since the frame guard stage number of the frame guard system used in a receiver of a mobile telephone is fixed, the frame synchronization tends to be easily in the out-of-frame state, which results in that the communication becomes unstable and thus a stable communication cannot be realized at all times.